The present invention relates to a radiation detector and, more particularly, to a direct-conversion type semiconductor radiation detector for measuring the photo energy of a radiation.
A detector, such as a spectrometer counter utilizing the external photoelectric effect of a photocell, has been known as a device for detecting a radiation such as a gamma-ray.
Recently, a direct-conversion type semiconductor radiation detector for transforming a photon energy directly into an electric signal has been developed by utilizing the internal photoelectric effect induced by the interaction between a semiconductor and a radiation.
FIG. 1 shows a bulk type semiconductor detector. In FIG. 1, two electrodes are so disposed on the side surfaces of the bulk type semiconductor detector as to face each other and extend along the photon-incident direction. A voltage is applied between both electrodes 5 and 6 of the semiconductor detector. When photons are applied to a semiconductor detector, pairs of electrons and positive holes 3 and 4 are generated in a semiconductor crystal by an internal photoelectric effect resulting from the energy of the photons incident on the detector. The energy of the photons is substantially proportional to the energy of the electron-positive hole pairs. When the electron-positive hole pairs are generated in the semiconductor crystal, electrons 3 move toward electrode 5 of positive voltage side, positive holes 4 move toward electrode 6 of earth side so that the electrons and the positive holes arrive at the electrode surfaces. Upon movements of the electrons and the positive holes, a signal current corresponding to the energy of the photons is generated in an external circuit connected between both the electrodes. This signal current is detected as an integration of a voltage signal by the external circuit. Thus signal charge arrived at the electrode surfaces has a correlation with the intensity of the radiation. Signal charge Qout arrived at both the electrodes is represented by the following equation (1): EQU Qout=Qe.lambda.e/D(1-e.sup.-X/.lambda.e)+Qh.lambda.h/D(1-e.sup.(D-X)/.lambd a.h (1)
where Qe, Qh: charge quantities of electrons and positive holes (Qe=Qh)
.lambda.e, .lambda.h: mean free paths of electrons and positive holes
D: distance between electrodes
X: distance between the positive voltage electrode and to the position where pairs of electrons-positive holes are generated.
Equation (1) is a general equation, in which the first term is charge quantity by the electrons, and the second term is charge quantity by positive holes. In actual measurements, carriers are captured in a defect, such as lattice defect, formed in a semiconductor detector of a room temperature operable type, having high quantum efficiency made of CdTe, HgI.sub.2 or GaAs. Since the probability of capturing positive holes is particularly high, most cases satisfy .lambda.h&lt;&lt;D. Thus, the energy of the photons is determined by only the quantity of generated electrons, because the charge by positive holes can be substantially ignored.
In such a case, the signal charge Qout is approximately represented by the following equation (2).
Qout=Qe.lambda.e/D(1-e.sup.-X/.lambda.e) (2)
In equation (2), it is understood that signal charge depends upon distance (X) between the position where the pairs of electrons and positive holes are generated, and the electrode of positive voltage side. Thus, charge Q varies according to the generated position. In other words, in conventional bulk type semiconductor detector, charge might be influenced by the distance (X), and signal voltage might also be varied.
Conventional bulk type semiconductor detector has energy-spectral characteristic of the photons as shown in FIG. 2.
FIG. 2 shows the relationship between the number of output signal counted by a radiation detector and the energy of its signal when photons having a predetermined intensity are continuously applied to a conventional semiconductor detector. In FIG. 2, an ordinate axis indicates the number of photons counted during a predetermined time, which represents the specific output value, and an abscissa axis indicates the value of an output signal corresponding to the energy of photons. It is understood from the spectral characteristic in FIG. 2 that a detection signal varies in the conventional detector. It is desired that a sharp peak should be presented only at the specific value corresponding to the energy of incident photons. In fact, as shown in FIG. 2, since the position of generating the electrons affects the output signal, a number of irregularities are observed, which cause energy resolving power to be remarkably reduced.